The present invention relates to apparatus for feeding and collecting webs or strips, especially webs or strips of photographic paper in a photographic printer or copier. More particularly, the invention relates to improvements in apparatus of the type disclosed in the aforementioned commonly owned copending application Ser. No. 364,080 by Steinberger et al now U.S. Pat. No. 3,904,145.
The application of Steinberger et al discloses apparatus which transport webs of photographic paper in a printer wherein the web is moved stepwise by advancing or feeding rolls serving to draw the web off a supply reel so that a portion or frame (having a length corresponding to or slightly exceeding that of a print) is held at a standstill at the copying station when the printer is actuated to make an exposure. The exposed portion of the web is convoluted on the core of a takeup reel which is driven by a motor in stepwise fashion. In order to insure that the take-up reel will accumulate tightly packed convolutions of exposed web, the apparatus further comprises a friction brake which is actuated (a) by the web or (b) by an electromagnet and grips the web between the copying station and the takeup reel while the latter continues to rotate due to inertia subsequent to stoppage of the motor. The length of the web can be in the range of several hundred meters, and the takeup reel is preferably devoid of flanges so that its core can collect relatively wide or relatively narrow webs. Thus, it is not necessary to adjust the takeup reel when the persons in charge decide to transport webs whose width deviates from the width of previously transported web or webs.
In the friction brakes of the type disclosed in the application of Steinberger et al, the friction coefficient between the components of the brake varies in dependency on factors which are often beyond the control of operators. Such factors include the speed at which one or more rigid components of the brake move relative to one or more stationary rigid components. The coefficient of friction decreases with increasing relative speed and reaches a maximum value when the relative speed is zero. This means that tensional stresses upon the web can reach a very high value, especially when the takeup reel already contains a large supply of convoluted material, because the kinetic energy of the takeup reel plus the kinetic energy of that portion of the web which is already convoluted thereon is converted into a force tending to stretch and eventually break the web when the latter is held against movement by the applied friction brake but the takeup reel continues to rotate in a direction to collect the web. The likelihood of breakage due to excessive tensional stressing cannot be eliminated by reducing the force with which the components of the friction brake grip the web upstream of the takeup reel because this could result in slippage of the web relative to the components of the brake. Such slippage would damage the photosensitive emulsion and/or would bring about pressure-induced exposure of the gripped web portion.